 I'm Tom Wellock, the historian at the NRC. The role of the historian at the NRC is to communicate with the public about the history of nuclear regulation through articles, books, and video presentations. My motivation for writing this book stems from the fact that before I was an historian, I was an engineer. 23 years ago I left the nuclear industry and became an historian. I was away for a long time teaching history and when I came back to the NRC what struck me was how much people talked about risk. Risk-informed regulation, probabilistic risk assessment. It was really a change from the industry that I knew in the 1980s and I was curious about that topic. How the history of probabilistic risk assessment, understanding, and quantifying risk was important to regulation of nuclear power. The most challenging aspect of writing this book was the sources. The NRC has no end of historical evidence that I could go through. It took me about eight years to research and write the book. The best part about writing this book was how far back in time people have tried to quantify the risk of a nuclear reactor accident. It used to be that this story started with what was known as the Reactor Safety Study which was published in 1975, but my story reached all the way back to the late 1940s. How hard engineers tried to quantify risk and the ways in which they tried to do it. They looked at everything from basic technology to the way human beings interact and might make mistakes. It was interesting just how much effort and resources were poured into quantifying risk. I think readers will be most surprised to learn about the way risk assessment in the nuclear industry influenced other agencies and issues outside the United States. I look at risk assessment at NASA, the Environmental Protection Agency, and also the way they tried to ensure that reactors in the post-Soviet world were safe in Central and Eastern Europe as well as the former Soviet Union. I hope readers learn from this book just how difficult these topics were for engineers, for policy makers, and for the public to come to terms with. And I hope what they also learn is about this is a very controversial topic and I hope they develop a certain amount of empathy and understanding for positions that they might not agree with, whether it's in the nuclear industry among critics of nuclear power or the regulators who tried to come up with a solution to the question how safe is safe enough.